This project focused on understanding the role of fluids in the process of continental rifting. Continental rifting is when continents begin to break apart due to tectonic forces pulling on the continents. Examples includes the Rio Grande Rift in the Western US, where the continent began breaking apart about 25 million years ago forming the volcanic activity in New Mexico and Southern Colorado. However, in this example the continent stopped breaking apart and the volcanic activity also stopped. Today in this region, the extension in the Rio Grande Rift has ceased and so has the volcanic activity. The East African Rift, on the other hand, is a continental rift where rifting is very active today. This rift stretches over 3000 km from the Gulf of Aden to Botswana in the South. Here rifting causes hot solid rock in the mantle to well up and melt then producing active volcanoes at the surface. 

In this project we aimed to understand the relationship between the fluids that are dominated by carbon dioxide in the mantle below the rift and the formation the rift. We collected numerous samples of waters in an active part of the rift in Tanzania and analyzed the content of carbon dioxide (CO2) and other gases in these waters. We found that the fluids in the active part of the rift are rich in carbon dioxide and that the other gases in these waters clearly point to the expected origin in the mantle. In contrary, when we sampled and analyzed the waters that emerged on the solid, unrifted part of the continent (the Tanzania craton) we did not detect any CO2. These observations, we then combined with models that address rift formation.  In our model, the carbon dioxide accumulates over many millions of years in solid form under the craton and over time moves sideways and upwards towards the Earth's surface. This process causes the melting of the surrounding rocks in the mantle and the formation of magmas that migrate upwards and result in the volcanic activity at the surface.

We therefore showed that accumulated carbon is mobilized by the initial rifting process and once the rifting process begins, this mobilization towards the surface triggers the formation of melts that then in turn further promote the rifting process and the associated volcanic activity. Our model is potentially widely applicable to other continental rifts in the geologic past.

Our broader impacts include the supervision of a local Tanzanian student at the University of Dar Es Salaam who used some of the data collected during this project for his M.Sc. thesis. This student was jointly supervised by the project principal investigators and a professor of geological sciences at the University of Dar Es Salaam. Additional broader impacts include the quantification of carbon dioxide emissions from an active rift. While these emissions are much smaller than current anthropogenic emissions, such emissions may have had climate effects in the geologic past (prior to when human activity dominated carbon emissions to the atmosphere). Another broader impact of the work is that we showed where carbon rich magma occurs in continental rift settings. These magmas and associated volcanoes are important because the contain valuable elements such as rare earth elements that are critical for use in modern society and that are the building blocks for key technological advances such as smart phones and high-strength magnets used in wind-power generators. Our work therefore, potentially enables future investigators to execute a more focused search for mineable quantities of such critical and rare elements.

Last Modified: 11/13/2022
Modified by: Tobias P Fischer